This invention relates to the field of video projection display and in particular to the use of a photo transistor for measurement of projected illumination.
In a projection video display, geometrical raster distortions result from the physical placement of the cathode ray display tubes. Such raster distortions are exacerbated by the use of cathode ray tubes with curved, concave display surfaces and the inherent magnification in the optical projection path. The projected image is composed of three scanning rasters which are required to be in register one with the other on a viewing screen. The precise overlay of the three projected images requires the adjustment of multiple waveforms to compensate for geometrical distortion and facilitate the superimposition of the three projected images. However, manual alignment of multiple waveforms is labor intensive during manufacturing, and without the use of sophisticated test equipment may preclude setup at a user location. Thus an automated convergence system is disclosed which simplifies manufacturing alignment and facilitates user location adjustment. An automated alignment system may employ raster edge measurement at peripheral display screen locations in order to determine raster size and convergence. Such raster edge measurement may be facilitated with a plurality of photo transistors.
A projection television display apparatus with an automated alignment system may advantageously employ raster edge measurement at peripheral display screen locations with a plurality of photo transistors. A method for determining raster positioning in a video projection display apparatus comprises the steps of detecting illumination by a first edge of a measurement image moving in a first direction. Detecting illumination by a second edge of the measurement image moving in a second direction, and averaging movement values related to the first and second edge illuminations of the detecting steps.
In a parallel sensor arrangement differences in image edge determination are advantageously precluded by sensing and detecting marker image edges with a marker that reverses sensing direction. In simple terms, motion direction of the marker block image is reversed relative to the sensor, which consequently reverses the leading and trailing marker image edges. Thus dissimilar edge determination is precluded. In more general terms, edge dissimilarities are obviated and accurate image edge sensing is obtained when image edges cause the sensor to transition from an unlit to lit condition.